1. Field of the Invention
This invention relates to a contour measuring device and, more particularly, to an optical system used to view the surface of a test specimen, such as blades and vanes used in a turbine engine, during the contouring process.
2. Description of the Prior Art
The measurement of a complex surface by taking a successive series of cross-section measurements is generally known as "contouring". Knowing the precise shape of a complex surface can be particularly important where the surface is interacting with a fluid, and hence accurate contouring is a necessary prerequisite to the efficient design of blades and vanes used in a turbine engine. One such optical inspection system is disclosed in U.S. patent application No. 715,557 filed on Dec. 17, 1976 and assigned to the same assignee as the present application. In the embodiment disclosed in FIG. 2, a first and second pair of mirrors are spaced apart about the collecting axis; this axis also coincides with the plane in which the turbine blade is moved. A focusing lens is located between the first and second pair of mirrors along the collecting axis. A beam of light from a laser is directed to the surface of a blade. The pairs of planar mirrors collect light scattered from the spot on the surface of the turbine blade and, after focusing by the lens, present an imaged spot to the diode array. An imaged spot from both the upper and lower surface of the test specimen is presented to the diode array. Variation in the thickness of the turbine blade causes corresponding vertical movements of the light spots on opposite sides of the turbine blade, and this, in turn, results in a proportional deviation of the imaged spot on the linear diode array. By electrically interrogating the incremental elements of the diode array for each step of the movement of the beam across the blade cross section, an electrical signal indicative of the contour of the cross section is derived.
Another optical inspection system is disclosed in U.S. patent application No. 751,558 filed on Dec. 17, 1976, now abandoned, also assigned to the same assignee as the present invention. The optical inspection system described in this application employs a pair of parallel mirrors and a beam splitter which are used to view the incident light beam on the surface of the turbine blade from two different directions. The beam splitter optically combines light from separate paths thereby allowing a contour to be taken close to either shroud of the turbine blade without remounting the test specimen.
Another optical contouring device is disclosed in U.S. Pat. No. 3,782,287 issued to T. Neeson on Jan. 1, 1974. In this system a test specimen is moved under a beam of light and the reflected image passes through a beam splitter, an objective lens, and a pinhole aperture.
U.S. Pat. No. 3,909,131 issued to J. Waters on Sept. 30, 1975, also assigned to the same assignee as the present invention, describe a slightly different concept for surface gaging. A collimated light beam is focused on the test specimen and the light scattered therefrom is collected through a lens and presented to a detector via a folding mirror.
Other techniques and apparatus for contouring complex surfaces are described in U.S. Pat. No. 3,174,392 issued to K. Rantsch on Mar. 23, 1965, U.S. Pat. No. 3,975,102 issued to A. Rosenfeld on Aug. 17, 1976, U.S. Pat. No. 3,894,802 issued to P. Higgens on July 15, 1975, U.S. Pat. No. 3,918,816 issued to G. Foster on Nov. 11, 1975, U.S. Pat. No. 3,898,007 issued to K. Wiklund on Aug. 5, 1975 and U.S. Pat. No. 3,898,583 issued to D. Shuey on Aug. 5, 1975.
Many of the hereabove identified prior art systems which employ a coherent source of light such as a laser exhibit a condition known as "speckle" resulting in spatial variation of intensity across the beam incident on the test specimen. As a result, when the incident beam is projected onto a diode array, these intensity variations create a nonpredictable response, and this reduces spot resolution when the elements of the array are electrically interrogated.
Some prior art optical inspection systems have lenses or other optical elements which are located in the plane of movement of the test specimen. This can be a particular problem when the test specimen is very long, such as the main rotor blade of a helicopter or the like, since such "on axis" optical elements limit the length along the test specimen which can be contoured.
Still other prior art systems employed conventional low cost spherical lenses and located these elements either "on axis" or "off axis" to collect light scattered from the incident spot. The conventional low cost type of spherical lens most often used in such systems exhibits certain inherent aberrations. These abberations introduce optical errors into the measuring device which limit accuracy.
Another problem occurs in the type of system in which light is collected from both sides of a test specimen simultaneously and then the imaged spots are presented to a single diode array of the charged-coupled type. This problem results from the fact that these spots are close together when measuring thin portions of the specimen, such as near the forward or trailing edge of a turbine blade. As the result of electrical charge "spill over", adjacent diode elements erroneously appear to be receiving light from the focused spot and electrical interrogation of the elements of the diode array can render erroneous data concerning the thickness of the test specimen at that point.
In some prior art systems in order to measure the complete contour of a turbine blade it was necessary to move either the illuminating optics or the viewing optics, or both, with respect to the test specimen so that the incident axis and the collecting axis always cross at he surface of the test specimen. Inherently, the movement of the optical elements require very accurate mechanical parts which can provide a readout of the position of the changes in the axis crossings. These systems are expensive, slower and generally have lower accuracy than the type of system with fixedly mounted optical elements.